Nedarim, Chapter Ten, Mishnah Six
This mishnah discusses a yavams rights to annul the vows of a woman who is awaiting yibbum or halitzah with him. To remind ourselves briefly, if a husband dies without children his wife is liable for either yibbum (levirate marriage) or halitzah (the release from levirate marriage) with his brother. Generally speaking, any brother may perform yibbum or halitzah.
1) If a woman waits for a yavam, whether for one or for two [yevamim]:
a) Rabbi Eliezer says: he can annul [her vows].
b) Rabbi Joshua says: [only if she waits] for one, but not for two.
c) Rabbi Akiva says: neither for one nor for two.
2) Rabbi Eliezer said: if a man can annul the vows of a woman whom he himself acquired, isnt it logical that can he annul those of a woman bequeathed to him by Heaven!
a) Rabbi Akiva said to him: No! If you speak of a woman whom he himself acquires, that is because others have no rights in her; will you say [the same] of a woman given to him by Heaven, in whom others too have rights!
b) Rabbi Joshua said to him: Akiva, your words apply to two yevamim; but what will you answer if there is only one yavam?
c) He (Rabbi Akiva) said to him (Rabbi Joshua): the yevamah is not as completely acquired to the yavam as a betrothed girl is to her [betrothed] husband.
Section one: There are three halakhic opinions in this section. According to Rabbi Eliezer, even if there are several brothers (yevamim) any one of them may annul her vows. According to Rabbi Joshua if there is only one yavam he may annul her vows but if there are more than one, neither may annul her vows. Rabbi Joshua believes that the connection between a yavam and his yevamah is strong enough that he may annul her vows but if there are two yevamim each prevents the other from doing so. Also, if there are two yevamim we dont know who will end up performing yibbum and therefore neither may annul her vows. According to Rabbi Akiva a yavam never has the right to annul his yevamahs vows.
Section two: In this section the three rabbis argue out their respective positions. Rabbi Eliezer reasons that in a normal marriage a woman is acquired by the man himself (acquisition is one of the typical ways by which the mishnah refers to marriage) but in yibbum Heaven bequeaths the yevamah to the yavam. In other words the relation between the yavam and the yevamah is created at the moment of the brothers death without any action by the yavam and therefore it is a stronger connection. If in the weaker acquisition the husband is allowed to annul vows, so too in the stronger acquisition.
Rabbi Akiva responds by pointing out that the two situations are not truly analogous. If there are two or more yevamim then each has some rights in her, because any of them can perform yibbum or halitzah with her. However, in cases of normal marriage, only the betrothed husband has rights in her, for only he will be able to marry her.
Rabbi Joshua points out that Rabbi Akivas refutation of Rabbi Eliezer holds true only if there is more than one yavam. If there is only one yavam he is the only one who has rights in her, and therefore he should be allowed to annul her vows. Why then does Rabbi Akiva say that he cannot.
Rabbi Akiva responds again, this time with a more fundamental difference between normal marriage and yibbum. The union between a betrothed couple is stronger than the unity between a yavam and a yevamah (remember this is before yibbum has been performed; after yibbum she is his wife in all regards). A betrothed woman who has relations with a man other than her betrothed husband is an adulteress and is technically liable for the death penalty. In that sense Jewish betrothal works like marriage. In contrast, a yevamah who has relations with a man other than her yavam is not considered an adulteress and has merely broken a less severe commandment, one not punishable by death. From here we can deduce that the bond of betrothal is stronger and therefore a betrothed husband may annul his betrothed wifes vows but a yavam may not do so for his yevamah.